This invention relates to braiding of the type typically constructed of monofilament or multi-filament yarns braided together to form sleeves for protecting, typically, automotive or aircraft electrical wiring or tubing to prevent or inhibit abrasion thereof.
Braiding of the aforementioned type is generally made from individual strands or collections of strands overbraided with adjacent strands or sets of strands and made from high-grade polymers such as nylon. For high performance applications where, particularly, the diameter of the monofiament needs to be small such as in the diameter range of 0.07 mm to 0.80 mm to meet specific cover and performnance criteria of the braid, a polyaryletherketone such as polyetheretherketone (known as "PEEK") is typically used either by itself or in combination with other monofilaments made from less expensive polymers. A known property of PEEK is its superior resistance to abrasion and this is why it is a preferred material in safety critical applications such as in automobiles and aircraft. However the cost of PEEK monofilaments has always tended to mean that the material is only used when no other, cheaper, material can meet the performance characteristics required, particularly with regard to abrasion resistance and light weight relative to the amount of cover provided by the braid when the monofilament is within the diameter range indicated above.
Of the less expensive high temperature melt spinnable fibre forming thermoplastics which are also suitable for use in making braiding there is polyphenylene sulfide (known as "PPS"), polybutylene therapthalate (known as "PBT") and polyethylene napthalate (known as "PEN") as well as polyimides (known as "PET") and aliphatic polyketones (known as "PK") which can all be made up into solid monofilaments with which a braided object such as a tubular sleeve can be constructed.
A braided tubular sleeve can easily be expanded by being compressed along the length of the tube so as to fit easily over wiring or tubing to be protected and then the sleeve can be pulled along its length so that its diameter is reduced to fit snugly around the wiring or piping. Although tensile strength in the braid is of importance in that it must be sufficiently strong to resist normal wear and tear, nevertheless provided the tensile strength is sufficient to make the individual strands of filament substantially recoverable for the purposes of acting as a braid, as required, the very high tensile strength afforded by solid monofilaments of thermoplastics are, to a large extent, unnecessary.
In U.S. Pat. No. 4,251,588 issued to Goetmann et. al. hollow polymer monofilaments are described which are used in paper-making belts to provide improved dimensional stability and flexibility. The filaments are described as being prepared according to customary techniques for making hollow monofilaments where the molten thermoplastic polymer is extruded through a vented orifice die into a quench medium, after which it is orientated by being stretched from about 3.4 to 6.0 times the original length, resulting in the monofilaments generally having a void content of about from 3% to 15% of their cross-sectional area. It is stated that with a void content of less than about 3% little benefit over solid monofilament is realised and with a void content in excess of 15% the monofilament tends to lose its substantially circular cross-sectional configuration too readily and flattens to a substantially void-free filament.
These findings are confirmed in U.S. Pat. No. 5597450 issued to Baker et. al. where in a woven, heat set fabric, for use in a paper making and like machine, at least a portion of the weft strands are hollow thermoplastic polymer monofilaments having a solidity in their undeformed cross-sectional area from about 50% to about 80%. The circumference of the hollow filaments is greater than or equal to the perimeter of the weft passageways they are to occupy in the fabric after heat setting, the stated advantage being to ensure that air permeability is both low and uniformly constant throughout the woven fabric. A further stated advantage is that, because some of the monofilaments are hollow they have less mass than comparably sized solid monofilaments such that their inertia is lower, thereby reducing problems associated with the acceleration and deceleration of large diameter monofilaments on high speed weaving looms.